Histamine is a compound possessing significant biological activity mediated by pharmacological receptors. Histamine has long been contemplated as a molecule having primarily negative biological effects. Recently, however, new uses for histamine as a powerful pharmaceutical agent have come to light. For example, histamine has been used in conjunction with interferon-alpha to activate NK cells (Natural Killer cells) in the presence of monocytes. See U.S. Pat. No. 5,728,378. To take full advantage of the therapeutic properties of histamine, it is necessary to obtain large quantities of the compound in a pharmaceutical grade.
Histamine occurs widely in nature as a result of putrefactive processes and a derivative, histamine dihydrochloride, is sold commercially for use as a standard in assays and as a component in certain allergy diagnostic kits. The source of this histamine is often a natural one and as such contains a variety of contaminants that render it unsuitable for pharmaceutical use. There are also synthetic protocols for the synthesis of histamine dihydrochloride known in the art.
Histamine dihydrochloride can be conveniently synthesized by exploiting the decarboxylation of histidine. Using this synthesis pathway, histidine is decarboxylated and subsequently treated to form the dihydrochloride salt form of the molecule. For example, Hashimoto et al., discussed the preparation of histamine using cyclohexenone as a catalyst for the decarboxylation of histidine. (Hashimoto, M., et al., Chemistry Letters, 893-896 (1986)). The Hashimoto, et al., paper reported the isolation of histamine dihydrochloride at a 95% yield, using 2-cyclohexen-1-one as the catalyst, from the reaction involving histidine and 1% v/v of 2-cyclohexen-1-one in 10 parts of refluxing cyclohexanol (26 hours). The Hashimoto method also teaches the use of toluene and HCl gas bubbled through the resulting decarboxylated solution to precipitate out and harvest the final histamine dihydrochloride product.
Attempts to reproduce the Hashimoto procedure to generate pharmaceutically pure amounts of histamine failed. Additional amounts of the catalyst were required to make the procedure operative and a substantial number of impurities were present in the final product. Moreover, those impurities were difficult to remove. In view of these results, it was found that the Hashimoto procedure is an unsuitable method for generating large quantities of pharmaceutically acceptable histamine.
The use of acetophenone as a catalyst for the decarboxylation of histamine has also been reported. We recreated the method described in the Japanese patent to Akimasa, et al., patent, Japanese Patent No. 05,255,204 (1983), and used 0.26 equivalents of acetophenone and 10 parts of diethylene glycol as the solvent for the decarboxylation reaction. Although the Akimasa et al. method was far more efficient in converting histidine to histamine, it failed to consistently yield a pharmaceutical grade product. Like the final product using the Hashimoto method, impurities were observed in the final product made using the Akimasa method during the HPLC analysis.
Although the conditions with acetophenone and diethylene glycol looked promising, there existed a problem related to the work-up. Both histamine free base and the dihydrochloride salt are readily soluble in water, therefore, it was difficult to utilize any extraction technique to separate the product from the diethylene glycol solvent, which was usually removed by a water extraction. Furthermore, the histamine dihydrochloride was also readily soluble in diethylene glycol, thus the direct isolation by filtration was also impossible.
The reaction conditions of Takano et al., involving pentan-3-one were also recreated. (Heterocycles, 6:1167 (1977)). The results from these experiments showed no improvement over the acetophenone conditions described above.
A consistent source of pharmaceutical grade histamine is required, especially in view of the new-found pharmaceutical applications for histamine. The standard methods used by the art wherein histamine is purified from natural sources, fail to yield histamine of a sufficiently high grade for pharmaceutical uses. Moreover, the synthetic methods practiced in the art also fail to yield histamine of a sufficiently high grade. Accordingly, there is a need in the art for an improved method by which to produce pharmaceutical grade histamine dihydrochloride.
The invention disclosed herein relates to the preparation of pharmaceutical grades of histamine dihydrochloride using a two step non-enzymatic synthetic method. One embodiment of the invention is a method for the synthesis of histamine dihydrochloride comprising: decarboxylating a L-histidine containing solution, whereby a histamine containing solution is formed in the absence of a decarboxylating enzyme; forming a histamine monohydrochloride containing solution from the histamine containing solution; and forming a histamine dihydrochloride containing solution from the histamine monohydrochloride containing solution.
One aspect of this embodiment further comprises triturating the histamine containing solution, for example, the histamine containing solution can be triturated with a methylene chloride solution. In another aspect of this embodiment, the histamine monohydrochloride containing solution is formed by addition of an effective amount of hydrochloric acid in an isopropanol solution. For example, the effective amount of hydrochloric acid is about 0.1 to 0.9 molar equivalents of hydrochloric acid to histamine free base. In another example, the effective amount of hydrochloric acid is about 0.6 molar equivalents of hydrochloric acid to histamine free base. Still another aspect of this embodiment further comprises the step of isolating a pharmaceutical grade of histamine dihydrochloride from the histamine dihydrochloride containing solution.
Another embodiment of the invention disclosed herein is a method for synthesizing a pharmaceutical grade of histamine dihydrochloride comprising: decarboxylating a L-histidine containing solution, whereby a histamine containing solution is formed in the absence of a decarboxylating enzyme; forming a histamine monohydrochloride containing solution from the histamine containing solution; forming a histamine dihydrochloride containing solution from the histamine monohydrochloride containing solution and isolating the histamine dihydrochloride from the histamine dihydrochloride containing solution.
In one aspect of this embodiment, the histamine dihydrochloride contains equal to or less than each of the following: 0.8% L-histidine HCl monohydrate, 0.1% individual chromatographic impurities, and 2% total impurities.